This invention relates to electronic cooking ovens in general, and more specifically to domestic cooking appliances for cooking foods by the application of energy in the microwave frequency range. The use of such ovens has become increasingly widespread largely due to the speed of cooking offered over conventional techniques.
Microwave ovens have heretofore had some characteristics of cooking performance that were less than satisfactory, expecially in the area of the eveness of the energy pattern throughout the oven capacity and it is well-known that microwave ovens frequently exhibit a pattern of "hot spots" or "cold spots" within the oven.
There are many causes of uneven cooking patterns and performance, and in substantial part of such patterns are determined by the method in which microwave energy is introduced into the cavity. Early ovens provided coaxial antenna projecting through the wall of the oven into the cooking cavity. Other arrangements provided for slotted waveguides which transmitted the energy from the magnetron to the cavity. Still other arrangements coupled the energy into a feed box, or intermediate zone between the waveguide and the cavity and added some type of rotating, energy reflecting device to aid in breaking up standing wave patterns.
It is common practice to use such a stirrer device in the oven cavity itself or in a feed box to change the number of modes present during an interval when food is being heated. A single stationary mode in a microwave oven cavity will exhibit itself as alternate hot spots and cold spots in the heated food. The hot spots are about 2.5 inches apart in an oven operating at 2450 MHZ. The purpose of the stirrer is to attempt to shift the position of the hot spot by changing the phase relationship of the waves that combine to form the single stationary mode.
While these techniques have provided some improvement, ideal performance has not been achieved. Moreover, because of these limitations, microwave ovens have largely been limited to cooking one type of foodstuff at a time.
The invention disclosed herein establishes new techniques for overcoming many of the performance limitations of prior art microwave ovens, especially as those limitations involve eveness of cooking pattern, magnetron to waveguide to cavity impedance matching, power coupling efficiency, and the like. Beneficial use has been made of certain microwave characteristics and techniques which, although known in other fields of microwave technology, are considered undesirable in those fields. One such characteristic which is known in the fields of radar and long distance microwave communications is called the "long lines effect", and equipment used in those fields is generally designed and constructed to eliminate the effect insofar as possible. I have discovered that those same effects can be intentionally designed into microwave ovens to produce suprisingly superior results.
The oven disclosed herein also departs from the conventions of the industry and supplies energy from the magnetron into the cooking cavity through each of two opposite walls, rather than from either the top or bottom of the cavity as is almost universally the case with ovens sold today. As a result, the cavity is horizontally rather than vertically polarized and the interference attributed to the food load in the oven is greatly reduced.